We have previously reported the transient appearance of an immunoglobulin (Ig) allotype (hidden allotype) in mice that were bred to exclude such an allotype. Our general aim is to test whether this phenomenon is due to a regulatory control mechanism of the immune system. Our first objective, to demonstrate unequivocally the presence of BALB/c Ig allotype in C.B mouse 1(congenic BALB/c mice that are homozygous for a marker chromosome carrying the C57BL Ig genes ; C.B mice are otherwise not known to be different from the BALB/c strain.) serum apart from possible cross-reactive BALB/c Ig idiotypes, has been achieved with the aid of our newly-adapted radioimmune assay. We have found that IgG1 and IgG1 and IgG2a of BALB/c allotype are produced transiently in C.B mice in amounts less than 0.2 percent of the total 7S Ig fraction (manuscript in preparation). This seems to reflect a problem in Ig regulation, but it is not clear whether we are dealing with too little (deficiency) or too much regulation (suppression). If the latter were true, it would suggest a possible connection between the phenomenon of hidden allotypes and allotype suppression, that is, a specific allotype may be hidden due to the presence of suppressor cells in normal cell populations; our goal for the coming year will be to investigate this possibility. BIBLIOGRAPHIC REFERENCES: Quantitation of mouse immunoglobulin allotypes by a modified solid-phase radioimmune assay. Bosma, M., Marks R. and De Witt, C. J. Immunol. 115: 1381, 1975. Suppression of immunoglobulin allotype production in adult congenic mice. Bosma, M.J. and Bosma, G.C. Nature (in press), 1976.